1. Field of the Invention
The present invention relates to a method of controlling transmission power in a CDMA mobile communication system, and more particularly to a method of controlling the uplink outer-loop transmission power for updating the reference value used in closed-loop transmission power control.
2. Description of the Related Art
FIG. 1 is a block diagram showing the configuration of a CDMA communication system. As shown in FIG. 1, this CDMA communication system is composed of mobile station (MS) 11, base transceiver stations (BTS) 112 and 113, base station controller (BSC) 14 for controlling these base transceiver stations 112 and 113, and mobile switching center (MSC) 15.
Mobile station 11 communicates with base transceiver station 112 and base transceiver station 113 by means of radio signals. Base transceiver stations 112 and 113 are connected to base station controller 14 by way of wired lines 16, and base station controller 14 is further connected to mobile switching center 15 by way of wired line 17. In FIG. 1, communication with mobile station 11 is realized by means of diversity hand-over, and signals from mobile station 11 are received at both base transceiver station 112 and base transceiver station 113 and then combined at base station controller 14. Signals from mobile switching center 15 are split at base station controller 14, transmitted simultaneously from base transceiver station 112 and base transceiver station 113, and then combined at mobile station 11.
In a CDMA mobile communication system, code division multiplexing technology enables simultaneous communication between a plurality of mobile stations and base transceiver stations on the same frequency. However, because communication between a particular mobile station and a particular base transceiver station can cause interference for other mobile stations, the mobile station and the base transceiver station constantly instruct the partner station to increase or decrease the transmission power so as to maintain the reception quality at its own station at a value that has been set in advance. This method of controlling the transmission power is referred to as “closed-loop transmission power control,” and the reception quality in closed-loop transmission power control is determined by, for example, the Signal-to-Interference power Ratio (hereinbelow abbreviated as SIR).
With reference to FIG. 2, a closed-loop transmission power control method in a CDMA communication system will be described. FIG. 2 shows radio signal formats in a W-CDMA communication system, these being radio signal format 21 of a downlink channel from a base transceiver station to a mobile station and radio signal format 22 of an uplink channel from a mobile station to a base transceiver station. These radio signal formats 21 and 22 contain pilot signals 23 and 25, and further, contain TPC signals 24 and 26 for instructing the partner station to increase or decrease the transmission power. The base transceiver station and mobile station use pilot signals 23 and 25 to measure the SIR of the communication, and use TPC signals 24 and 26 to instruct the partner station to increase or decrease the transmission power in order to approach the target value that was instructed beforehand by the base station controller. More specifically, the mobile station calculates the SIR from pilot signal 23 in interval 27 and causes the result to be reflected in TPC signal 24, and the base transceiver station similarly calculates the SIR from pilot signal 25 in interval 28 and causes the results to be reflected in TPC signal 26.
In a CDMA mobile communication system, a user signal at the time of hand-over is combined in the base station controller, which is the host device of a base transceiver station. This type of hand-over method is referred to as a “soft hand-over.” The base station controller determines the reception quality of the user signal following combining by soft hand-over, determines whether the reception quality satisfies a target reception quality level that has been set in advance for the service, such as voice service, that is being provided to the user, and updates the SIR target value for the appropriate base transceiver station so as to maintain the target reception quality level. This method of controlling the transmission power is referred to as “outer-loop transmission power control,” and the reception quality that is measured by the base station controller is, for example, Block Error Rate (BLER).
FIG. 3 shows the construction of base station controller 14 that is shown in FIG. 1. As shown in FIG. 3, base station controller 14 is provided with BLER measurement unit 31, data adder 32, distribution unit 33, selective synthesizer 34, target SIR calculation unit 35, and central control unit 36.
In this base station controller 14, users' signals from base transceiver stations 112 and 113 undergo a synthesizing process in selective synthesizer 34 for signals from the same mobile station. In this synthesizing process, the user signals are subjected to a statistical process over a plurality of radio frames. The BLER of the user signal after carrying out this combining process is then measured at BLER measurement unit 31.
The BLER that was measured at BLER measurement unit 31 is then compared with the target BLER that was set by means of central control unit 36 at target SIR calculation unit 35, and the value of the target SIR of the uplink signal that is the reference in closed-loop transmission power control is then calculated based on the results of comparison. More specifically, target SIR calculation unit 35 performs a process for increasing the target SIR value of the uplink signal when the BLER that has been measured by BLER measurement unit 31 is equal to or lower than the target BLER that has been set by means of central control unit 36, and performs a process for decreasing the target SIR value of the uplink signal when the BLER that has been measured by BLER measurement unit 31 is greater than the target BLER that has been set by means of central control unit 36.
The target SIR value of the uplink signal that has been calculated by target SIR calculation unit 35 is contained in data that are transmitted by data adder 32 to each base transceiver station, and this target SIR value is transmitted to, for example base transceiver stations 112 and 113 by way of distribution unit 33. Base transceiver station 112 and base transceiver station 113 subsequently execute control of the closed-loop transmission power in accordance with the target SIR value of the uplink signal that has been reported from base station controller 14.
The uplink (in the direction from a mobile station to a base transceiver station) radio signal outer-loop transmission power control system in a CDMA communication system of the prior art will be next explained with reference to FIG. 4. In FIG. 4, the communication with mobile station 11 takes place during a diversity hand-over where mobile station 11 is communicating with base transceiver station 112 and base transceiver station 113. The uplink user signal from mobile station 11 is received by base transceiver station 112 and base transceiver station 113, and this user signal is transmitted from each of base transceiver stations 112 and 113 to base station controller 14.
Upon receiving the user signals from base transceiver stations 112 and 113 in step 61, base station controller 14 performs a synthesizing process in selective synthesizing unit 34 as shown in FIG. 3. In this process of combining user signals, a statistical process is carried out over a plurality of radio frames in steps 62 and 63, following which the uplink signal target SIR value is calculated in target SIR calculation unit 35 in step 64. The results of this calculation are then reported to base transceiver station 112 and base transceiver station 113 by means of an uplink signal target SIR report message. Base transceiver stations 112 and 113 subsequently execute closed-loop transmission power control using, as a reference, the uplink signal target SIR value that was reported by means of the uplink signal target SIR report message. Simultaneously execution of the above-described closed-loop transmission power control method and outer-loop transmission power control method in a CDMA mobile communication system maintains the transmission power of mobile stations and base transceiver stations at optimum levels.
However, the outer-loop transmission power control method involves a considerable amount of processing time due to the use of the block error rate as the reception quality as well as the measurement of the quality of the user's data following combining in the base station controller, and as a result, the uplink signal target SIR value that serves as the reference value in closed-loop transmission power control cannot be set to a base transceiver station at high speed. This inability results in inadequate management of instantaneous changes in the communication environment such as sudden changes in the fading environment of radio communication, and a consequent potential for the occurrence of deterioration in communication quality such as interruptions to communication.